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In the last few years, the current density of Bi-2212 wires has been greatly improved by filament densify technologies. One further step toward applications in fusion magnets is to design a high current cable, which must overcome the drawbacks of Bi-2212 wires: brittleness and complex heat treatment process. In this article, react-and-wind flat cables are proposed. With react-and-wind technique, a larger number of choices of jacket and quench protection materials are available including stainless steel and copper, both of which are widely used for Nb3Sn and NbTi superconducting cables but are not directly usable for wind-and-react Bi-2212 cables due to their reaction during heat treatment. Two wires, representing, respectively, modest and top performance of the state-of-the-art wires, are considered. Several cable layouts are studied in terms of dimension, compaction rate, and transverse mechanical pressure. Materials for quench protection and jacket materials are also discussed.

State-of-the-art high field solenoids make use of hybrid designs exploiting the superior high field performance of High Temperature Superconductors (HTS) in the innermost region. The benefits of a hybrid Central Solenoid in a pulsed tokamak like the EU DEMO can be two-fold: either to reduce its outer radius (which would result in a reduced overall size and cost of the tokamak), or to increase the generated magnetic flux (which could extend the plasma burn time and possibly increase the power plant efficiency). In the framework of the pre-conceptual design studies for DEMO coordinated by EUROfusion, a hybrid Central Solenoid is proposed based on ten layer-wound sub-coils using HTS, Nb3Sn, and Nb-Ti conductors respectively for the high, medium, and low field sections. The design exploits the flexibility of layer winding by grading both the superconductor and the stainless steel cross sections in each sub-coil, which has the potential for space and cost savings. Mechanical analyses have identified fatigue as the main design driver for the EU DEMO Central Solenoid. Possible alternatives to reduce the sensitivity of the proposed design to fatigue are currently under investigation.

The present study aims to reduce the outer radius of the central solenoid (CS) with respect to its nominal size specified by EUROfusion for a maintained CS magnetic flux. A reduced outer CS radius would allow the reduction of the overall size and cost of the DEMO magnet system. The proposed outline design of the winding pack for the CS1 module is based on layer winding. To achieve the same magnetic flux in a CS coil of significantly reduced outer radius the peak magnetic field at the CS conductors needs to be substantially increased. The use of high-temperature superconductors is therefore envisaged in the highest field sections of the CS coil. It is planned to use react & wind Nb3Sn conductors for intermediate field sections and NbTi at the lowest fields. In order to make a most economic use of the superconductors the proposed winding pack design considers a superconductor grading.

In the last 10-15 years, the most common strategy in the development of High Temperature Superconducting (HTS) cable for magnets has been to imitate Low Temperature Superconducting (LTS) cable designs. However, requirements for LTS materials are not valid for HTS materials, which are extremely stable. For example, non-twisted multifilamentary Bi-2223 tapes have been successfully used in several magnets. This paper review stability and analyse inductance and AC losses in non-twisted stack of HTS tapes. Numerical calculations show that twisting has negligible effect on inductance variations in a stack of tapes. Regarding AC losses, any magnet built with coated conductors have larger losses than LTS ones, because of the aspect ratio and large width of the tape. If a wide tape is replaced by a non-twisted stack of narrow tapes, losses and residual magnetisation could be reduced. In contrast with multifilamentary wires, twisting a stack of tapes reduces losses only marginally. Therefore, cables composed of non-twisted stack could be designed to have losses comparable to the one of twisted stack concepts. Few examples of large cables for fusion applications are discussed. Designs based on non-twisted stacks can be simpler, more robust and cost effective than twisted ones., Comment: 20 pages, 10 figures, submitted to Cryogenics

CRPP hosts two unique conductor test facilities SULTAN (SUpraLeiter TestANlage) and EDIPO (European DIPOle). They allow the test of high current superconductors in high magnetic fields (SULTAN 11 T, EDIPO 12.5 T). In both facilities sample currents up to 100 kA can be supplied by means of a NbTi transformer. Presently the facilities are upgraded for the test of high current high-temperature superconductor (HTS) samples. For HTS conductor testing at temperatures between 20 and 50K, the heat flux between the HTS sample under test and the NbTi transformer needs to be limited to around 10W per conductor leg by means of an HTS adapter connecting them. The second required upgrade is the supply of intermediate temperature helium (20-50 K) to the HTS test conductor. It is mandatory that the helium gas coming from the HTS conductor under test can be returned to the cryoplant as cold gas (T < 20K). To reach this goal a tube-in-tube heat exchanger has been manufactured in which 4.5K helium coming from the cryoplant is in counter flow with the warm gas leaving the HTS test conductor.

In this study, we investigated in field properties of high-Tc superconductor (HTS) tapes electrically as well as mechanically. Metallic reinforced Bi-2223/Ag superconductor tapes were used in this work. The critical current (Ic), which is highly dependent on the magnetic field, was measured using fields applied parallel and perpendicular to the tape surface. The conductor was exposed to fields of up to 28 T at 4.2 K. In order to confirm the performance of tapes in practical NMR operation, Ic was also measured around 2 K. The conductor was wound on a large diameter bobbin and mechanical properties were then measured as the coil current was increased. Finally, we verified the conductor properties from coil Ic and maximum stress/strain of it.

Superconductors for high field magnets should have high mechanical strength, high Young modulus as well as high critical current density. The optimization of these properties is very important in the design of a magnet and its conductors. In case of high field solenoids, the main factor limiting the coil current is the mechanical properties of the conductors. A new optimization procedure for conductors is introduced, starting from the competition between the transport properties and the mechanical properties. The balance between mechanical and transport properties are calculated depending on the coil diameter and on the applied magnetic field, and examples of applications to Bi2223 tapes are presented.

Magnetic flux concentration is a very important technique for the effective generation of high magnetic fields. We propose the use of the diamagnetism of a high T c bulk superconductor (HTS bulk) instead of ferromagnetic materials such as Ho and Dy. We fabricated a magnetic field concentrator using Gd-Ba-Cu-O bulk. The essential point was slits to suppress the current along the circumference. The concentrator was cooled at the center of a superconducting magnet with liquid helium. By increasing the external field to 1.00 T, a magnetic flux density of 3.20 T was obtained at the center of the concentrator. At an external field of 2.00 T, a field of 5.65 T was also obtained. A magnetic lens using HTS bulk was successfully demonstrated. The experimental results were well explained by numerical analyses assuming HTS bulk as a perfect conductor.

A new calorimeter has been built with the special purpose to determine the distribution of Tc in industrial superconducting wires. Specific heat measurements have been carried out on a series of multifilamentary Nb3Sn wires, using a long relaxation technique. The advantage of this technique consists in the fact that the measurement is performed in presence of the Cu-Sn matrix, i.e. the filaments are measured under the same stress conditions as under operation, i.e. under the same state of mechanical precompression. In addition, the Tc distribution is obtained for the whole sample volume, ruling out shielding effects. The deconvolution of the data in the region of the superconducting transition was used for getting the precise distribution of Tc, which in turn allows a determination of the Sn distribution across the filaments. These data confirm previous TEM measurements showing a Sn gradient inside of the filaments of bronze route processed Nb3Sn wires. The Tc distribution has been determined in Nb3Sn wires processed by bronze route, internal Sn and powder-in-tube technique. Based on this information, the various processing parameters can be varied to get narrower Tc distributions at transition temperatures closer to 18 K.

The addition of Ti and/or Ta to commercial multifilamentary Nb3Sn wires is commonly used to increase their upper critical field, and thus the critical current density at high fields. We have compared the critical parameters (Tc and Bc2) and the critical current densities Jc at 17 and 21 T for several multifilamentary quaternary (Nb,Ti,Ta)3Sn wires prepared by the bronze route process. The goal of this work is to establish a correlation between the Ti content and the superconducting properties (Tc, Bc2, Jc) of these wires. We have found that Ti additions do not lead to a further increase of Tc and Bc2 in quaternary (Nb,Ta,Ti)3Sn wires with respect to ternary (Nb,Ta)3Sn wires. On the other hand, the highest Jc was measured in quaternary (Nb,Ta,Ti)3Sn wires (1.5 at.% Ti with respect to the Nb–7.5% Ta cores), where Ti was added to the bronze matrix. It was found that the wire configuration has only a small effect on Tc and Bc2.

Multifilamentary superconducting Nb3Sn wires are widely used for industrial applications. Wires processed by the Bronze Route technique are characterized by a large number of filaments Nb1−xSnx, with 0.18≤x≤0.25, corresponding to a distribution of Tc between

We have measured the field (B) and strain (e) dependence of the critical current (Ic) in Nb3Al conductors, prepared by the rapid-heating, quenching and transformation (RHQT) technique. The measurements were made using a modified Walters spring (WASP) at fields up to 21 T, and at 4.2 K. The measurements at the highest field have given a clear field dependence of the Ic versus e curve, which has enabled us to evaluate the wire's absolute characteristics over wide field and strain ranges. The data were well fitted with empirical scaling laws with specifically given constants for the Nb3Al wires. The strain sensitivity of the transformed Nb3Al wire is also compared with a low-temperature diffusion processed wire.

The strain dependence of not only the critical current (Ic) but also the n-value in technical Nb3Al wires prepared with the rapid-heating, quenching and transformation (RHQT) method has been measured with the modified Walters spring (WASP), which allows us to measure the voltage–current relation down to less than the 0.1 µV cm−1 criterion, an important requirement in view of the applications like NMR spectroscopy operated in a persistent current mode. The measurement was made in fields up to 17 T and at 4.2 K, for several kinds of Nb3Al wires, including stabilized wires and wires with a tantalum matrix.

We have developed a device for the electromechanical characterization of 80 cm long technical superconducting wires carrying up to 1000 A at 4.2 K up to 17 T. Our device is based on the concept of the Walters spring (WASP), which allows us to measure long length wires (up to 80 cm) and to obtain an absolute measurement of the applied strain value. It is thus possible to measure the voltage–current relation of technical superconducting wires and tapes down to 0.1 µV cm−1, an important requirement for the characterization in view of applications like NMR high field magnets which require persistent mode operation. In this contribution, we report on Ic measurements versus strain at fields up to 17 T for industrial Nb3Sn and BSCCO: the voltage–current relation as a function of strain and magnetic field has been measured over three decades of electric field.

We report on a new device for measuring the variation of the critical current as a function of the uniaxial strain in Nb/sub 3/Sn industrial wires. This device offers the possibility of measuring large critical currents (up to 1000 A) on long superconducting wires (about 0.8 m), thus allowing to use the 0.1 /spl mu/V/cm criterion. The WASP is built out of a titanium alloy, enabling strains up to 1.4% at 4.2 K to be applied. In our design, and in contrast to the original Walters spiral, the superconducting wire lies in a machined groove and is only soldered to Cu contacts at both ends. The strain applied on the wire is directly measured by means of strain gauges. With this new device, the variation of I/sub c/(/spl epsiv/) for Nb/sub 3/Sn wires with rectangular cross section as large as 1.56/spl times/1.02 mm/sup 2/(I/sub cmax/=708 A at 13 T) have been successfully measured for the first time.

Several (Nb,Ta,Ti) 3 Sn multifilamentary wires with different Ti contents (up to 0.6 wt.%), but identical configuration have been processed using the bronze route and NbTa/NbTi composite filaments. The wires were manufactured by a sequence of three extrusion steps using a Cu 15.4 Sn Osprey bronze as matrix. The wires of 1.06 mm diameter with 14641 filaments of 4.4 μm size were reacted by various heat treatments, ranging from 600 to 730 °C. Critical current density ( J c ) measurements on samples in a coil geometry have been performed up to 17 T at 4.2 K, yielding to 195 A mm −2 at 17 T. Upper critical magnetic fields ( B c2 ) up to 28.2 T were estimated by Kramer extrapolation. The variation of the critical temperature ( T c ) as well as of the n factor were determined. The effect of various Ti contents was analyzed with respect to the various superconducting parameters, especially in view of applications at fields >20 T.

Rare Earth based coated conductors are very promising conductors for the construction of high field solenoids. Development of coil winding technique was started at the Tsukuba Magnet Laboratory: coils of various sizes (from 18 mm to 220 mm of an inner diameter) have been fabricated by layer-winding without impregnation in order to test the mechanical and superconducting properties. In the thin 220 mm diameter coil the Hoop stress reached over 500 MPa at 130 A in 14 T background field, without degradation of the superconducting properties. In the small diameter coils (18 mm of inner diameter) the tape was insulated using enamel to minimize the cross section and to enhance the coil current density. The highest field generated was 2.1 T in a background field of 18 T (total field was 20.1 T) at 4.2 K and the coil current density was 240 A/mm 2 .

Bronze processed Nb3Sn superconducting wires are today still considered as the best candidates for nuclear magnetic resonance (NMR) magnets operating in persistent mode at high magnetic fields. The metallurgical treatment of Nb3Sn bronze wires with additives like Ta and Ti is quite difficult to further improve because the correlation between the additive and its effect on the superconducting properties, which do not only depend on its concentration but also on the whole variety of fabrication parameters. The aim of this work is to clear up the different effects of the internal (Nb-Ta / Nb-Ti filaments) and external (Cu-Sn-Ti bronze) Ti doping method on the superconducting properties and to analyze their possible reasons [1].

The production of superconducting Nb3Sn multifilamentary wires with optimized critical currents requires a detailed knowledge of various processes, involving both the mechanical deformation and the reaction kinetics. The physical properties of the Nb3Sn phase are briefly reviewed, the precise knowledge of their variation as a function of the Sn content being essential for the optimization of the critical current density, Jc. The variation of the transition temperature Tc, the upper critical field, Bc2(0), and the normal state electric resistivity ρo, as a function of the Sn content, β, in the binary system Nb1−βSnβ is carefully analyzed. The effect of Ta, Ti and Ta + Ti additives to Nb3Sn on the physical properties is discussed in detail. Low temperature specific heat measurements are introduced for determining the Tc distribution inside Nb3Sn filaments while avoiding shielding effects. The microstructure of the superconducting phase in Bronze Route and Internal Sn Diffusion processed wires is studied, taking into account the unique microstructure of Bronze Route filaments, comprising an equiaxed and a columnar grain region, their areas being comparable. The Sn content increases gradually, from 18 to 22 at.% in columnar and from 22 to 25 at.% in equiaxed grains. Taking into account the equiaxed grains only in Bronze Route wires, it is found that the pinning force density FGB is essentially the same as in the superconducting part of Internal Sn and PIT wires. The lower values of the overall FGB in Bronze Route wires is due to the presence of columnar grains, with lower Tc and Bc2. The presence of columnar grains also explains the deviation from linearity of the Kramer rule in Bronze Route wires. The mechanism leading to the variation of Jc vs. e, where e is the uniaxial applied strain, is correlated to the elastic tetragonal distortion of the A15 phase in the filaments, caused by the matrix precompression or by Lorentz forces. The behavior of Jc (e) is found to show systematic differences between Bronze Route and Internal Sn processed wires. Possible reasons for the stronger variation of Jc (e) up to 21 T in Internal Sn wires are discussed.

The harmonics of the AC magnetic susceptibility as a function of the temperature have been measured on Nb and NbTi polycrystalline samples, at various amplitudes of the AC magnetic field, without a DC field. The data have been analysed in terms of Cole-Cole plots, i.e. the imaginary part as a function of the real part. A similar AC field dependence in the height of the maximum of the first harmonics Cole-Cole plots has been observed for both materials. The experimental results have been compared with numerical curves obtained by integrating the nonlinear diffusion equation for the magnetic field inside the sample with different flux creep models. It has been proved that the AC field amplitude dependence have to be included in the existing models in order to reproduce the behaviour of the experimental data. Finally, a quantitative agreement has been achieved by supposing different vortex dynamical regimes tuned by changing of the AC field amplitude. (c) 2007 Elsevier B.V. All rights reserved.

We report on experimental evidence of the presence of a universal behavior in the field-temperature phase diagram of type II superconductors, characterized by a phase transition in the vortex matter between the disordered and the Bragg glass phase. The experimental detection of a peak effect phenomenon has been proved to be strictly connected to the existence of this phase transition. In this paper, we show an observation of a peak effect in the compound ${\mathrm{Nb}}_{3}\mathrm{Sn}$, by using first harmonics of the ac magnetic susceptibility. Peak effect has been detected at fields between 3 and $13\phantom{\rule{0.3em}{0ex}}\mathrm{T}$, whereas it is not observable at higher fields. This seems to be in contrast with the theoretical predictions of such a phase transition at all fields and, therefore, with the predicted universality in the magnetic behavior of the type II superconductors. Nevertheless, by measuring the third harmonics of the ac susceptibility, this phase transition has been detected up to our highest available field $(19\phantom{\rule{0.3em}{0ex}}\mathrm{T})$, thus demonstrating the necessity of the higher harmonics analysis in studying these topics and moreover proving the validity of the theoretical predictions.

A comparative study of magnetic behavior in a DC magnetic field up to 19 Tesla has been undertaken on different Nb3Sn samples, i.e. single crystals, polycrystal samples and multifilamentary wires. From the 1st and 3rd harmonics of the AC magnetic susceptibility a peak effect was experimentally observed in the single crystal and in the polycrystal, whereas this phenomenon is absent in the measured wires. The corresponding H vs T phase diagram reveals a different behavior between wires on one hand and polycrystalline and single crystal samples on the other. In particular, from the 3rd harmonics it has been observed that vortex thermally activated flux creep phenomena are relevant in the wires, whereas the static critical state models are more appropriate to describe single crystals., Comment: 9 pages, 9 figures, to be published on Advances in Cryogenic Engineering, Vol. 51-52 (CEC-ICMC05, Keystone, Colorado, 28 August-3 September 2005)

We report on experimental evidence of the presence of a universal behavior in the field-temperature phase diagram of type II superconductors, characterized by a phase transition in the vortex matter between the disordered and the Bragg glass phase. The experimental detection of a peak effect phenomenon has been proved to be strictly connected to the existence of this phase transition. In this paper, we show an observation of a peak effect in the compound Nb3Sn, by using first harmonics of the ac magnetic susceptibility. Peak effect has been detected at fields between 3 and 13 T, whereas it is not observable at higher fields. This seems to be in contrast with the theoretical predictions of such a phase transition at all fields and, therefore, with the predicted universality in the magnetic behavior of the type II superconductors. Nevertheless, by measuring the third harmonics of the ac susceptibility, this phase transition has been detected up to our highest available field (19 T), thus demonstrating the necessity of the higher harmonics analysis in studying these topics and moreover proving the validity of the theoretical predictions.

Fe/MgB2 tapes have been prepared starting with pre-reacted binary MgB2 powders. As shown by resistive and inductive measurements, the reduction of particle size to a few microns by ball milling has little influence on Bc2, while the superconducting properties of the individual MgB2 grains are essentially unchanged. Reducing the particle size causes an enhancement of Birr from 14 to 16 T, while Jc has considerably increased at high fields, its slope Jc(B) being reduced. At 4.2K, values of 5.3*10^4 and 1.2*10^3 A/cm^2 were measured at 3.5 and 10 T, respectively, suggesting a dominant role of the conditions at the grain interfaces. A systematic variation of these conditions at the interfaces is undertaken in order to determine the limit of transport properties for Fe/MgB2 tapes. The addition of 5% Mg to MgB2 powder was found to affect neither Jc nor Bc2. For the tapes with the highest Jc values, very high exponential n factors were measured: n = 148, 89 and 17 at 3.5, 5 and 10T, respectively and measurements of critical current versus applied strain have been performed. The mechanism leading to high transport critical current densities of filamentary Fe/MgB2 tapes based on MgB2 particles is discussed., Presented at ICMC 2003, 25-28 May 2003

Mono- and multifilamentary MgB2/Fe tapes and wires with high transport critical current densities have been prepared using the powder-in-tube (PIT) process. The fabrication details are described. The effect of powder grain sizes and recrystallization temperature on jc has been investigated. At 25K and 1 T, jc values close to 105 A/cm2 were measured, while jc of 106 A/cm2 were extrapolated for 4.2K/0T in our monofilamentary tape. MgB2/Fe tapes exhibit high exponential n factors for the resistive transition: n ~ 80 and 40 were found at 5 T and 7 T, respectively. The highest transport jc values obtained so far in MgB2/Fe wires with 7 filaments were 1.1 * 105 A/cm2 at 4.2 K and in a field of 2 T, which is still lower than for monofilamentary tapes. Improved deformation and recovering processing is expected to lead to higher jc values., 4 pages, 8 figures; Submitted to ASC 2002

Bronze processed Nb3Sn superconducting wires are today still considered as the best candidates for nuclear magnetic resonance (NMR) magnets operating in persistent mode at high magnetic fields. The metallurgical treatment of Nb3Sn bronze wires with additives like Ta and Ti is quite difficult to further improve because the correlation between the additive and its effect on the superconducting properties, which do not only depend on its concentration but also on the whole variety of fabrication parameters. The aim of this work is to clear up the different effects of the internal (Nb-Ta / Nb-Ti filaments) and external (Cu-Sn-Ti bronze) Ti doping method on the superconducting properties and to analyze their possible reasons [1].

The measurement of the actual thermal strain, , in a cable in conduit conductor (CICC) of is of paramount importance to predict the CICC performance in operation starting from the strand scaling laws for the critical current density . The behavior of the critical temperature as a function of the axial strain at constant field, , is known from the literature and is accurately described by interpolative scaling laws. In the scope of this work, the critical temperature as a function of the applied field, , is measured for the CICC in-situ and for the free standing filaments used for the cable manufacture. These results are compared with the curve to draw the thermal strain in the CICC. A small AC field is generated at the sample and its inductive response is recorded. At the superconducting transition the change from a diamagnetic to a paramagnetic response is observed. The experimental assessment of the in a CICC through measurements is the first step toward the quantitative evaluation of the irreversible degradation in CICC, which is estimated from the discrepancy of the predicted vs. actual CICC performance. This paper describes a novel method for the in-situ measurement of the in a large CICC, including the monitoring of during the electromagnetic cyclic loading test campaign in SULTAN of the ITER conductors.

During the test of ITER conductors at nominal conditions, a degradation of the current sharing temperature has been observed after electromagnetic cyclic loading. Rather than focusing on the microscopic phenomena at the origin of the degradation, the degradation process has been studied from a statistical point of view, analysing the whole dataset of ITER conductors: average degradations have been calculated after various cycle numbers. Possible correlation with the strand critical current has also been analysed. In order to model the evolution of has been correlated with the reliability against cracks formation; the reliability is in turn defined according to a two parameters Weibull distribution. The validity of this approach has been tested with the so-called “Weibull plot”. The Weibull exponents were calculated for several ITER conductors and were found to be lower than one, indicating that the failure rate, which may be regarded as the formation rate of fractures in the filaments composing the strands, is decreasing with the number of cycles.

Knowledge of the actual Nb3Sn filaments' thermal strain, epsilon(th), in a cable-in-conduit conductor (CICC) is essential to predict the CICC performance in operation starting from the strand scaling laws for the critical current density J(c)(B, T, epsilon). To obtain a measurement of epsilon(th) under relevant conditions, i.e. at low temperature and with the mechanical constraints of a long length section of CICC, the critical temperature as a function of the applied field, T-c(B), is measured by an inductive method for the CICC in situ and for the freestanding filaments used for the cable manufacture. To deduce the thermal strain in the CICC, the T-c(B) results are compared with the T-c(B, epsilon) curve. Starting from the susceptibility curves measured for both the CICC and the filaments, it is possible to compute the T-c distribution in the CICC using a deconvolution algorithm. The first results of T-c measured on two CICCs in the SULTAN test facility suggest a broad distribution of thermal strain, peaked at negative strain, which remains almost constant during the cyclic loading. From the knowledge of both the thermal strain distribution and the actual CICC performances, it will be possible to discriminate between reversible and irreversible degradation in Nb3Sn CICC.

A device has been developed to measure the critical current of long length superconductors under uniaxial strain up to 1000A and up to 21T. The latter is based on a modified Walters spring (WASP) where zero applied strain can be precisely controlled. The length of the investigated conductor is on the order of 1m with a typical gauge length between the voltage taps of about 0.5m. This facilitates the measurement of critical currents at an electric field criterion as low as 0.01μV∕cm, which is particularly important for superconductors to be used in magnets which are operated in the persistent mode. The operation of the WASP was simulated by finite element calculations indicating that the radial stress (contact pressure) of the superconductor is less than 5% of the axial stress. The performance of the probe is demonstrated for several Nb3Sn conductors with round and rectangular cross section, respectively, as well as for a Bi-2223 tape.

In the EDIPO test facility, superconducting cables with a current carrying capacity up to 100 kA can be tested in a background field of 12.5 T. A NbTi transformer, operated at ???4.5 K, provides the sample current of up to 100 kA. The high-temperature superconductor (HTS) adapter, which is similar to an HTS current lead, connects the HTS sample under test and the NbTi transformer and limits the heat flux between them to less than 20 W at a warm end temperature of 50 K. Helium of 4.5 K and 10 bar supplied by the refrigerator is warmed up to temperatures up to 60 K by means of heaters and a counter flow heat exchanger (HEX). The heat exchanger ensures that the warm helium leaving the HTS sample can be returned as cold gas with less than 20 K to the refrigerator. The commissioning of the HTS adapter and the heat exchanger has been performed together with the test of a 60 kA class HTS cable manufactured at CRPP. The results of the commissioning of the HEX and the HTS adapter are presented.

The present work is performed within the framework of the EUROfusion DEMO project. Previously, it was demonstrated that for a maintained magnetic flux the use of HTS conductors at highest magnetic field in a layer-wound CS coil would allow the reduction of its outer radius by around 0.5 m as compared to the DEMO reference design using only Nb3Sn conductors. Alternatively, the superior high field performance of HTS conductors can be used to maximize the magnetic flux of a CS coil with a given outer diameter, which could significantly prolong the duration of plasma burn phase and thus the overall power plant efficiency. In the present study, the maximization of the magnetic flux, generated by a layer-wound CS coil of fixed outer radius, is considered. HTS conductors are envisaged to be used at highest field, while Nb3Sn and NbTi are foreseen to be used in intermediate and low field layers, respectively. The inner radius of the CS coil is optimized with respect to the generation of a maximum flux taking into consideration the superconductor properties, the hoop stress and the axial stress. In order to provide reasonable starting values for the finite element analysis, first a simplified model with layer-dependent current densities, however, without stainless steel grading will be considered. In the final outline design of the CS coil a superconductor and stainless steel grading will be implemented.

The measurement of the actual thermal strain, epsilon th, in a cable in conduit conductor (CICC) of Nb3Sn is of paramount importance to predict the CICC performance in operation starting from the strand scaling laws for the critical current density J(c)(B,T,epsilon). The behavior of the critical temperature as a function of the axial strain at constant field, T-c(B, epsilon), is known from the literature and is accurately described by interpolative scaling laws. In the scope of this work, the critical temperature as a function of the applied field, T-c(B), is measured for the CICC in-situ and for the free standing filaments used for the cable manufacture. These results are compared with the T-c(B, epsilon) curve to draw the thermal strain in the CICC. A small AC field is generated at the sample and its inductive response is recorded. At the superconducting transition the change from a diamagnetic to a paramagnetic response is observed. The experimental assessment of the epsilon th in a CICC through T-c measurements is the first step toward the quantitative evaluation of the irreversible degradation in Nb3Sn CICC, which is estimated from the discrepancy of the predicted vs. actual CICC performance. This paper describes a novel method for the in-situ measurement of the T-c in a large CICC, including the monitoring of T-c during the electromagnetic cyclic loading test campaign in SULTAN of the ITER conductors.

An existing Nb3Sn laboratory magnet generating a magnetic field of 12 T is intended to be upgraded to 16 T by means of the use of a high temperature superconductor (HTS) insert coil. An outline design of the HTS insert coil is presented. In the design, the aspects of the maximum achievable operation current, the required copper cross-section to ensure a hot spot temperature below 200 K and the resulting forces and stresses have been considered. The length of the insert coil has been selected in such a way that the field uniformity will be better than 1% within a sphere of 3 cm diameter. The protection of the whole magnet system (LTS & HTS insert) is briefly described. (C) 2012 Published by Elsevier B.V. Selection and/or peer-review under responsibility of the Guest Editors.

Coated conductors are promising material for the construction of high field magnets, but the slow propagation velocity of the normal zone makes the protection against quenches a critical issue. The quench protection system for a small insert coil under development at CRPP has been studied. The voltage drop and the hot spot temperature during the quench have been estimated using analytical expressions; the cross section of the copper stabilizer, the voltage threshold and the time constant were set as parameters. Detailed calculations have been carried out using the Cryosoft code THEA. The copper cross section available from the tape manufacturer is enough to ensure that the hot spot temperature remains below 200 K, but only in conjunction with a sensitive quench detection system (voltage threshold lower than 100 mV) and a fast dumping time (time constant shorter than 100 ms). These requirements can be fulfilled in the small insert coil under construction, because the inductance is about 50 mH and because the coil can be quickly discharged independently from the main magnet. The passive protection circuit consists of dump resistor and diodes. In case of longer time constants (few seconds), a larger copper cross section (comparable to the one used in low temperature superconducting magnets) is required in order to maintain the temperature within safe values.

The commissioning of the European DIPOle (EDIPO) test facility at the Center for Research in Plasma Physics in Villigen was completed in Spring 2015, and the first test of a 60-kA/12-T high-temperature superconductor (HTS) was carried out in June 2015. User specifications are issued to allow external users to prepare their own sample matching the interface of the test facility. EDIPO offers a background magnetic field up to 12.35 T in a rectangular test well with a cross section of 89 mm $\times$ 138 mm (sample dimension). The homogeneous high field length is 910 mm (±1%) or 680 mm (±0.5%) at full current. A set of small copper coils provides a superimposed ac field with amplitude up to ±0.3 T and frequency up to 1 Hz. A superconducting transformer provides steady-state operating current for the sample up to 100 kA. The cooling circuit of the sample supplies supercritical helium at 10-bar inlet pressure. The mass flow rate, which is adjustable by cryogenic valves, is up to 8 g/s in each branch of the sample at 4.5 K. The operating temperature ranges from 4.5 to 60 K (at high temperature, the range of mass flow rate is reduced). To test samples at temperature higher than 10 K, an HTS “adapter” connects the sample to the NbTi transformer plates, which must remain cold. In such case, the length of the sample is reduced from 3600 mm (typical low-temperature superconductor sample) to 2900 mm (HTS sample with adapter).

Quench protection is a critical issue for magnet using high-temperature superconducting (HTS) materials such as REBCO. For cable-in-conduit conductor (CICC) with twisted REBCO tapes, which is considered for high-current HTS conductor for fusion magnet, the anisotropic critical current density (J(c)) of tape and complex conductor layout will further complicate the quench behavior. The CryoSoft code THEA is used to simulate the quench behavior of various subscale CICCs in view of a quench experiment in SULTAN facility. The CICCs are composed of twisted tape stack assembled into copper shell (strand). It has been found that neglecting the anisotropy of J(c) in quench simulation will significantly underestimate the hot spot temperature by about 40 K. Therefore, it could leave potential risk to damage the magnet. It is also shown that the thermal contact resistance between strand and the steel jacket (the conduit) plays a very important role on quench behavior. The hot spot temperature can be reduced by more than 100 K if the thermal resistance is reduced by two order of magnitude. On the other hand, variations of interstrand electrical and/or thermal resistance have only very little influence (few kelvin).

During the test of ITER conductors at nominal conditions, a degradation of the current sharing temperature has been observed after electromagnetic cyclic loading. Rather than focusing on the microscopic phenomena at the origin of the degradation, the degradation process has been studied from a statistical point of view, analysing the whole dataset of ITER conductors: average degradations have been calculated after various cycle numbers. Possible correlation with the strand critical current has also been analysed. In order to model the evolution of has been correlated with the reliability against cracks formation; the reliability is in turn defined according to a two parameters Weibull distribution. The validity of this approach has been tested with the so-called "Weibull plot". The Weibull exponents were calculated for several ITER conductors and were found to be lower than one, indicating that the failure rate, which may be regarded as the formation rate of fractures in the filaments composing the strands, is decreasing with the number of cycles.

Previous studies indicated that the use of high-temperature superconductors (HTS) in the highest field allows us achieving the required magnetic flux in the central solenoid (CS) of the European DEMO at a reduced outer diameter, which would enable a reduced overall size and cost of DEMO. The proposed winding pack design of the CS1 module is based on ten layer-wound subcoils using an HTS, react and wind Nb3Sn, and NbTi conductors in the high, medium, and low field sections, respectively. The design utilizes a stainless steel and a superconductor grading leading to different overall current densities in each of the ten subcoils. Both, the hoop stress and the vertical loads are taken into consideration for determination of the required stainless steel cross-section in the winding pack. The stainless steel grading has been found to have an even more pronounced effect on the overall subcoil current densities than the superconductor grading. A preliminary estimation of the space required for the precompression structure is also reported.

In the last few years, the critical current densities of long commercially available REBa2Cu3O7-x (RE-123, where RE represents Y or a rare earth element) coated conductors have reached values of 250 A/cm-width at 77K and zero applied field. Even higher values of 600 A/cm-w (77 K, B = 0) have been demonstrated in shorter lengths. The attractive features of the use of these high-T-c superconductors (HTS) are operation temperatures above 20K and/or magnetic fields higher than those envisaged for the ITER TF coils. Possible operation conditions for HTS fusion magnets have been studied taking into consideration the possible further improvements of RE-123 coated conductors. Investigations of stability and quench behavior indicate that stability is not a problem, whereas quench detection and protection need attention. Because of the high currents necessary for fusion magnets, many tapes need to be assembled into a transposed conductor. The qualification of HTS conductors for fusion magnets would require their test at magnetic fields of 11 land currents well above 10 kA. The possibilities to test straight HIS conductor samples in SULTAN have been considered. For a test at 4.5 K, only the development of a low resistance joint between the HTS conductor under test and the NbTi transformer of SULTAN would be necessary. Tests up to 20K would require that the HTS sample is connected with the NbTi transformer by a conduction-cooled HTS bus bar of large thermal resistance similar to the HIS module of a current lead. HIS conductor tests at temperatures around 50K would be possible with modified cryogenics. (C) 2013 Elsevier B.V. All rights reserved.

High temperature superconductors (HTS) may become in future an option for the superconducting magnets of commercial fusion plants. At the Swiss Plasma Center (SPC) the R&D activity toward HTS high current, high field cables suitable for fusion magnets started in 2012 and led in 2015 to the assembly of the first 60 kA, 12 T prototype conductor. The cable concept developed at the SPC is based on the principle of 'soldered, twisted stacks' of REBCO tapes. The required number of stacks is assembled in a cored flat cable, cooled by forced flow of supercritical helium. The sample environment of the test facility at SPC has been upgraded with a HTS adapter and a counter-flow heat exchanger to allow testing the HTS sample in a broader range of temperature (4.5 K-50 K) using the existing, NbTi based superconducting transformer and the closed loop refrigerator.